1. Field of the Invention
This application relates to apparatus and methods for stripping dissolved gases from water. More particularly, it concerns the degassification of water containing dissolved gases, especially effluent from ozone disinfection of water to obtain potable water.
2. Description of the Prior Art
Ozone is becoming more and more widely used as a disinfection agent for potable water because it has much lower potential for producing trihalomethanes than chlorine now commonly used. It is produced from the oxygen in air or from pure oxygen by passing the air or oxygen through an electrical plasma discharge. (see U.S. Pat. Nos. 996,561 and 4,865,749.)
Not all of the oxygen is converted to ozone so the gas coming from ozone generator will usually contain about 1 to 8% ozone depending on whether air or oxygen is used as the source gas and the power level of the plasma discharge in the generator.
When disinfecting with chlorine, the pure gas is used and the quantities normally used dissolve readily in the water. Because of its solubility, there is little danger that high concentrations of chlorine gas will be present in the air above the water. Although the water storage tanks or basins are covered, the vents need no special consideration because of the chlorine content of the water. When ozone is used as a disinfectant, this is no longer true and special vent provisions must be employed.
The methods of applying ozone are different from those used for chlorine because ozone is only part of the total gas volume used. Usually the ozone-air or ozone-oxygen mixture is admitted to the bottom of a basin referred to as the ozone contact chamber. Gas dissolution is achieved utilizing a network of piping equipped with gas diffusers (see U.S. Pat. Nos. 3,865,039; 3,945,918 and 4,076,617). The small bubbles of gas produced through the diffusers rise through the water in the contact chamber and the component gasses dissolve to essentially their saturation constant for the ambient conditions.
Because ozone is a toxic and corrosive gas, and not all of it and the other component gases are dissolved, special provisions must be made for the containment and removal of the ozone not dissolved. The contact chambers must be equipped with a gas tight cover to allow collection of the off gas and its discharge through a device capable of removing its ozone content.
Ozone contact devices are designed to provide essentially ideal conditions for its dissolution. As a result, each of the component gasses dissolve to essentially their saturation concentration. Later, as the water passes through other portions of the treatment systems, the equilibrium conditions (temperature, pressure) of the water may change. Hence, some of the dissolved gas can come out of solution, resulting in "air binding" of filter beds, pumps, piping, or other equipment. If ozone is added at the beginning of the treatment process, very small gas bubbles released can interfere with the sedimentation process by having tiny bubbles attach themselves to suspended particles causing them to float rather than settle as desired.
In addition to the above considerations, retrofitting an existing water treatment plant with an ozone contact basin can change the hydraulic gradient through such treatment plant. Hence, there may not be sufficient change in elevation to allow the water to flow through the plant without additional pumping.
A substantial number of water treatment plants are changing the disinfecting agent used from chlorine to ozone. For such plants to continue to operate at their best efficiency, it is necessary to reduce the dissolved gas content of the water below the level that causes performance problems in treatment processes. This "gas stripping" can be accomplished by several methods. To insure the dissolved gasses are below saturation level, it is necessary either to heat the water (impractical) or allow it to come in intimate contact with an ambient pressure environment at less than atmospheric pressure. One of the more usual solutions is to pass the water through a vacuum degassifier, i.e., a closed vessel operating at less than atmospheric pressure. The fluid, in this case water, is usually sprayed into such vessel or distributed over packing installed in the vessel to provide the large liquid surface film required for efficient gas transfer.
Although these vacuum degassifiers provide the desired results they are costly to build, install and operate. The vessels are of some height, about 20 feet or more. Pumping is required to elevate the water to the top of the vessel. If spray nozzles are used for efficient distribution of the water, additional pressure losses are generated so additional pump pressure is necessary. Large volumes of water are required to satisfy the requirements of a city, resulting in appreciable equipment size and pumping costs. Hence, there exists a need for a method of reducing the concentration of dissolved gases to the desired level without appreciably affecting the hydraulic gradient in the plant and provide improvement over present practices and substantially reduce pumping costs.
The need to degass water in water treatment plants is not restricted to ozone treated water. Thus, water treatment plants that do not use ozone for disinfection may frequently experience air concentration in their influent as high as 130% of equilibrium saturation. Under such circumstances, filter runs can be reduced 30-50%. Thus, air binding problems in the filter media occurs whenever saturation reaches 108-110%. This is caused by the fact that whenever water that is supersaturated with dissolved gases goes through a filter, the pressure drop through the filter media causes these dissolved gases come out of solution. When this occurs small bubbles are formed that stay within the interctices of the filter media and restrict water flow.
The standard technique for reducing gas saturation, namely, dropping the water 4-6 feet over a weir, results in a high head loss which can not be accepted in many existing plants. Hence, a need exists to enable influent water to be economically degassed without creating any substantial head loss. The present invention addresses this problem and supplies new apparatus and methods to help solve it.